Hello,
meanwhile I created my own test with direct comparison of mask with
the related opacity-gradient for such a simple sample and my results
for tests on my Linux machines (Debian lenny or sarge) are:
Correct behaviour for Opera 9/10, adobe-plugin, Gimp (2.2), Inkscape
Slightly wrong behaviour (as already mentioned in the discussion) for
Gecko 1.9, Batik/Squiggle 1.7, KSVG (if they use the same or similar
graphical library in Linux, this could be an explanation for the wrong
behaviour, if it does not appear for other OS).
Completely nonsense already for the gradient in webKit (aurora 0.4),
mask ignored; mask ignored too in Gecko 1.8 or Opera 8.